1. Field of the Invention
This invention relates to aqueous ink formulations for ink jet printing which contain carbon black products as pigment.
2. Discussion of the Related Art
Ink jet printing is a non-impact process wherein droplets of ink are produced and deposited on a substrate such as paper, transparent film, or textile material in response to an electronic signal. Typical ink jet printing systems are continuous stream or drop-on-demand type. In continuous ink jet systems, ink is emitted in a continuous stream under pressure through at least one orifice or nozzle. The stream is perturbed causing it to break up into droplets at a fixed distance from the orifice. At the breakup point, the droplets are charged in accordance with digital data signals and passed through an electrostatic field. The field adjusts the trajectory of each droplet to direct it to a gutter for recirculation or to a specific location on a recording medium to create images. In drop-on-demand systems, a droplet is expelled from an orifice directly to a position on a recording medium by pressure generated in accordance with digital data signals. A droplet is not formed or expelled unless it is to be placed on the recording medium. The drop-on-demand system has several advantages over the continuous system in that it requires no ink recovery, charging, or deflection.
There are three basic types of drop-on-demand ink jet systems. One type has an ink filled channel with a nozzle on one end and a piezoelectric transducer near the other end which produces pressure pulses. The relatively large size of the transducer prevents close spacing of the nozzles, which may limit print quality. Physical limitations of the transducer result in low ink drop velocity and slow printing speed. The low drop velocity may also diminish tolerances for drop velocity variation and directionality.
A second type of drop-on-demand system is known as a thermal ink jet or bubble jet printer. The major components are an ink-filled channel with a nozzle at one end and a heat generating resistor near the nozzle. Printing signals create an electric current pulse in a resistive layer within each ink passageway, causing the ink in the immediate vicinity to evaporate, creating a bubble. Some of the ink in the channel is forced out through the orifice as a propelled droplet due to bubble expansion. Thermal or bubble ink jet printers produce high velocity droplets and allow very close spacing of nozzles, which results in a high quality of print.
The third type of drop-on-demand ink jet device is known as an acoustic ink printer. This printer utilizes a focused acoustic beam formed with a spherical lens illuminated by a plane wave of sound created by a piezoelectric transducer. The focused acoustic beam exerts pressure on the surface of the liquid, resulting in the ejection of small droplets of ink onto an imaging substrate.
Conventional inks for ink jet printers generally comprise a colorant such as dye which is soluble in a vehicle of water or a mixture comprising water and a water-soluble or water-miscible organic solvent. However, dyes have several disadvantages when used in ink jet inks. Dyes, being water-soluble or soluble in a water and organic mixture, may redissolve and run when exposed to moisture or water. Dye images may smear and rub off on contact with felt pen markers or upon being rubbed with a finger. Dyes also exhibit poor light stability, including fluorescence, sunlight, and ultraviolet light. Inks comprising soluble dyes may also exhibit clogging of the jetting channels due to solvent evaporation and changes in the dye's solubility, dye crystallization, and the presence of impurities. Dye-based ink may also exhibit poor thermal and chemical stability which could result in poor print quality. The dye may also bleed or diffuse into pores of the paper, thus causing poor print quality and low optical density of the image. Specialty paper may be necessary for certain ink jet inks containing a dye. In addition, many of the dyes contained in inks may be potentially mutagenic.
Pigments have also been used as colorants in ink jet inks, either as substitutes for, or in combination with, dyes. Pigments offer properties superior to dyes in areas such as waterfastness, lightfastness, image density, thermal stability, oxidative and chemical stability, compatibility with other colorants, and compatibility with both coated/treated and plain papers. Additionally, pigments are generally nonmutagenic. Pigments used in ink jet inks include carbon black, titanium dioxide white, cobalt blue (CoO--Al.sub.2 O.sub.3), and chrome yellow (PbCrO.sub.4). Carbon black, which absorbs in the infrared region, may be used for bar code reading.
The major problem with the use of such pigments is dispersion. Pigment particles such as carbon black generally start in a clumped or agglomerated state. To prepare ink jet inks, however, the carbon black must be dispersed and stabilized in that form because the extent of dispersion directly affects ink jet printing characteristics such as ejectability, print quality, optical density, and the like. Additionally, since the nozzle openings of thermal or bubble type ink jet printers are typically about 40-60 micrometers in diameter, it is critical to ensure that the ink jet ink does not clog or plug these openings. Thus, it is necessary to make the pigment particles as small as possible and preferably reduce the carbon black to individual aggregates. Small pigment particles are also less prone to settling during storage and therefore further contribute to the stability of the carbon black dispersion.
In light of these requirements, conventional aqueous pigment-based ink jet inks generally contain an aqueous ink vehicle, a pigment, a dispersant, and a humectant to prevent drying of ink or the clogging of orifices. Further additives such as biocides, binders, salts, driers, penetrants, surfactants, and the like may also be included.
Dispersants are materials that can bind to the pigment with one part of a molecule, while the other is attracted to the vehicle. A dispersant typically coats the pigment particles and then attracts a coating of the vehicle, which allows the coated particles to disperse in the vehicle. Clumping and agglomeration of the pigment particles are therefore minimized due to a steric or electro-steric repulsion caused by the protective coverage.
The conventional steps for preparing a pigment-based ink jet ink, as well known in the art, are as follows: First, the pigment must be milled to a small size in the presence of the solvent and the dispersant, by any of a variety of mechanical dispersion processes, including a liquid jet interaction chamber, a media mill, a ball mill, a roll-mill, a speed line mill, a homogenizer, and a sand grinder. Without the milling step, the dispersant will be ineffective. The pigment dispersion may be diluted further with the vehicle to obtain a desired concentration. Finally, some additional ink components, such as humectant, viscosity control additive, biocide, fungicide, pH adjuster, anti-corrosion agent, and the like, may be added to the pigment dispersion. Further dilution with the vehicle may also be made. It may also be necessary to remove larger particles by filtration and/or centrifugation.
For general discussion on the properties, preparation, and uses of aqueous inks, see The Printing Manual, 5th Ed., Leach et al., Eds. (Chapman & Hall, 1993).
The majority of time and cost is therefore spent on adequately dispersing the pigment in the aqueous medium. The desire to ease and improve dispersion of pigments is reflected in the patents discussed below.
U.S. Pat. No. 5,184,148 discloses an ink jet ink comprising an aqueous liquid medium containing acid carbon black and a water-soluble resin having a weight average molecular weight within the range of 3,000 to 30,000. The resin acts to improve the dispersion stability of the carbon black in the liquid medium. Other additives such as surfactants, defoaming agents, preservatives, dyes, and the like, may also be included.
U.S. Pat. No. 4,597,794 discloses an ink jet recording process which uses an ink prepared by dispersing fine particles of a pigment into an aqueous dispersion vehicle containing a polymer having both a hydrophilic and a hydrophobic construction portion. The hydrophilic portion constitutes a polymer of monomers having mainly polymerizable vinyl groups into which hydrophilic portions such as carboxylic acid groups, sulfonic acid groups, and sulfate groups have been included. Examples of the monomers to which hydrophilic groups are attached include styrene and its derivatives, vinylnaphthalene and its derivatives, and .alpha.,.beta.-ethylenic unsaturated carboxylate of a C.sub.8 -C.sub.18 aliphatic alcohol. The hydrophobic portion of the polymer adsorbs to the pigment and the hydrophilic portion faces out from the pigment to bond with water or another aqueous solvent. This mechanism produces improved dispersion of the pigment. The ink compositions disclosed may also include additives such as surfactants, salts, resins, and dyes.
U.S. Pat. No. 4,530,961 discloses an aqueous dispersion of carbon black grafted with hydrophilic monomers of alkali or ammonium carboxylate bearing polyacrylates, which may be used for manufacturing ink jet inks. The carbon black is treated in water with a water-soluble peroxide, and after peroxidation, a water-soluble acrylic monomer and more peroxide are added. This process results in simultaneous homogeneous free radical polymerization of the monomer and grafting onto the carbon black. This composition may also contain surfactants, wetting agents, dyes, or other additives.
U.S. Pat. No. 5,281,261 discloses an ink containing carbon black particles having a polymeric vinyl aromatic salt either chemically grafted or adsorbed onto their surfaces. The salt may simply be adsorbed to the surface of the carbon black due to similar hydrophobicity of the carbon black and the aromatic groups of the salt. If the salt is grafted to the carbon black, it is not the aromatic portion of the salt which is directly grafted to the carbon black. Instead, a polymerizable olefinic group on the salt interacts with the carbon black to graft the salt thereon. Additionally, as with typical ink jet inks, additives such as surfactants, wetting agents, dyes, and the like, may be present.
The above U.S. Patents and article are incorporated herein by reference.
While the use of pigments such as carbon black in ink jet inks represents an alternative to dyes, there remains a need for a pigment that can be stably dispersed in its smallest particulate form without undue processing and cost.